1. Field of the Invention
The present invention relates to a conductive adhesive used in a field of packaging of electronic elements. Furthermore, the present invention relates to a packaging structure including a substrate and an electronic element that are electrically connected with each other via the conductive adhesive.
2. Description of the Prior Art
Due to a recent trend of environmentally-friendly sensitivity, controls over lead included in solder alloys will be imposed in a field of electronic packaging, and thus, establishment of a lead-free packaging technique, i.e., a technique to join electronic elements with a lead-free material is an urgent necessity. The lead-free packaging technique includes packaging using mainly a lead-free solder or a lead-free conductive adhesive. Conductive adhesives have been noted particularly in the technique since they are expected to provide merits such as joint flexibility and lower packaging temperatures.
A typical conductive adhesive is prepared by dispersing conductive particles in a resin-based adhesive ingredient (binder resin). In general, packaging of an element is carried out by applying a conductive adhesive on a substrate electrode, packaging the element, and subsequently curing the resin. In this way, the joints are adhered with the resin, and the conductive particles are contacted with each other due to the contraction of the resin so that the conductivity at the joints is secured. Since the curing temperature of the conductive adhesive resin is about 150xc2x0 C. and this is lower than a solder melting point of 240xc2x0 C., such a conductive adhesive can be used for inexpensive parts having inferior heat resistance. Moreover, since the joints are adhered with a resin, they can respond flexibly to distortion caused by heat and/or external force. Therefore, the conductive adhesive has a merit that less cracks will occur at the joints when compared with a solder having alloy joints. For the above-mentioned reasons, a conductive adhesive is expected as an alternative to solder.
However, a conductive adhesive is inferior to a solder alloy in the packaging reliability in a state being joined with a multipurpose electrode element and with a substrate. In general, base metals such as solder alloys and Cu are used for terminal electrodes of circuit boards and of electronic elements. When electronic elements and circuit boards having terminal electrodes of base metals are packaged with conductive adhesives, the connection resistance is increased remarkably under an atmosphere with high temperature and high humidity. A major factor regarding the increasing connection resistance in a packaging structure with a conductive adhesive is that the base metal used for the electrodes corrodes in the presence of moisture. In other words, particles of a metal such as silver in the conductive adhesive contact with moisture entering the base metal electrode so as to form a kind of electric cell, and thus, the base metal electrode having relatively low potential is corroded. Therefore, multipurpose cells should be replaced by electrodes of expensive metals such as Au or Pd in order to secure moisture-proof reliability when a conductive adhesive is used.
In order to solve the above-mentioned problems, the present invention provides a conductive adhesive and a packaging structure that can maintain a moisture-proof reliability even when a multipurpose base metal electrode is used.
A conductive adhesive according to the present invention is used to electrically connect an electronic element and a substrate, and the conductive adhesive includes first particles having a standard electrode potential equal to or higher than a standard electrode potential of silver and also second particles having a standard electrode potential lower than a standard electrode potential of silver.
Since the conductive adhesive contains second particles having a low potential and the second particles are subject to sacrificial corrosion, corrosion in the electrodes of the electronic element and of the substrate electrode is controlled.
Furthermore, the present invention provides a packaging structure including an electronic element and a substrate in which the electronic element and the substrate have electrodes connected electrically via the above-mentioned conductive adhesive. In the packaging structure, the second particles corrode and the resulting ingredient can exist as at least one kind of compound selected from an oxide, a hydroxide, a chloride and a carbonate.
Moreover, the present invention provides a packaging structure including an electronic element and a substrate, and the electronic element and the substrate have electrodes connected electrically via a conductive adhesive. This conductive adhesive includes particles having a standard electrode potential that is equal to or higher than a standard electrode potential of silver. A coating of a metal compound having a standard electrode potential higher than that of the particles is formed on a surface of at least one electrode having a standard electrode potential lower than that of the particles and composing the electronic element or the substrate.
In the packaging structure, a coating of a metal compound having a high potential is formed on a surface of the electrode having a low potential in order to control corrosion in the electrode.